Supernatants from human mixed lymphocyte cultures (XRF (xenogeneic reconstitution factor)) restored the in vitro anti-SRC PFC response of T-cell deficient murine spleen cell cultures (spleen cells from congenitally athymic nude mice, TxB mice, and adherent spleen cells from normal mice). Using TxB spleen cells, it was shown that XRF was required for only a short time at the beginning and at the end of the incubation period, which suggested a bi-modal effect of the supernatant. It was found that XRF was totally incapable of restoring the PFC response if the residual T-cells were killed by anti-T cell serum. The PFC response of the mouse B-cells, in the presence of XRF, was restored by virgin mouse T-cells which, in the absence of XRF, were completely unable to reconstitute the response. Analysis of XRF by gel filtration revealed at least two peaks of sub-optimal activity which eluted with molecules of 10- 14,000 Daltons (Factor 1) and 45- 75,000 Daltons (Factor 2). Each of the components were incapable of independently restoring the PFC response but synergistically interacted to restore the immune response. The results suggest that induction of the B-cells to the formation of PFC requires: 1) Factor-1 mediated activation of the residual T-cells which provide an initiator function to the B-cells and 2) Factor-2 induced terminal differentiation of the PFC precursors.